1. Field of the Invention
The invention relates generally to earth-boring rotating cone drill bits and, more particularly, to drill bits having structures aimed at improved drilling rate and extended life span.
2. Description of the Prior Art
The basic design for a rotating cone drill bit is described in a patent filed in 1933, Scott et. al. “Three Cone Bit,” U.S. Pat. No. 1,983,316 (1934) and hasn't substantially changed or been substantially improved in concept since that time.
Rotating cone drill bits are used to drill wellbores for, e.g., oil and gas explorations. The most common types of rotating cone drill bits are three-cone rotating cone drill bits, which have three substantially cone-shaped cutter elements rotating on solid journals retained by ball bearings about their respective legs which are three segments which are fabricated into the bit body. The rotations of the cones are slaved by the rotation of the drilling string or mud motor or electric motor attached to the bit body portion (threaded pin end) of the rotating cone drill bit. Each cone has a plurality of inserts or teeth that disintegrate the earth formation into chips while the cones are rotating. Other types of drill bits, such as drag bits, also exist. In a drag bit, the cutting structures co-rotate with the drill string or mud motor or electric motor.
There are several factors which have limited the lifetime, durability and performance of drill bits as have been implemented in this conventional design over the last seven decades. A nonexhaustive listing of some of the inherent problems of the conventional rotating cone drill bit, which continue to this day, are listed below.
Problem areas have included the premature failure of the journal bearing which supports the cones as they rotate and the ball bearings that rotate between the journal and the cone retaining the cone.
One cause of such failures has been the leakage of abrasive drilling fluids and solids through the leg shirttail to cone shell gap into the bearings through the failed rotating seal caused by debris intrusion.
Another limitation of performance has arisen because of the loss of mud nozzles, obstruction of the hole bottom by debris inadequately cleared by the restricted mud flow, and the creation of hydraulic dead spots under the cones.
Bit lifetimes have been limited by the loss of cutting inserts and/or failure of cones due to loss of material in thinned areas of the cone shell.
Penetration rates have been limited due to inherent limitations on the cutter volume and cutting structure design which could be obtained on the cones, insufficient hydraulics, a faulty cone retention system, sealing the bearing, bearing properties, and a small bearing contact area causing high unit loads reducing the weight on bit.
Mud flows from the mud nozzles has been deflected and lost efficiency due to unavoidable interference from the cones and cutting structures, causing inter alia debris to be pushed back underneath the cones to be recut.
Cones are subject to wobble and gimbal as the bearing, which is poorly retained in position by the means of Scott's 1934 patented ball bearing retention design which wears out quickly resulting in a tapered, out-of-gage well bore section that must be re-drilled, and cutting inserts that become chipped, broken and/or dislodged.
Wobble of the cones as their bearings wear out which causes the cones to move in and out on their axes pumping grease out of the bearing and sucking or drawing mud into the bearing resulting in accelerated bearing wear, accelerated bearing wear is also caused by high unit loads and poor metallurgy which results in overheating and cone loss causing premature drill bit failure.
The retention balls in the bearing “brinell” the ball races like a ball peen hammer, accelerating cone loss and is one of the causes of premature failure of the bearing before the end of the wear-life of the cutting structures.
The ball retention design for retaining the cones on the journals removes material from the cone cross section further weakening the cone shell.
In insert type bits the cones utilize cutting inserts with differing grip depths, profiles, and grip diameters in order to be accommodated on the cone shell thereby rendering inserts vulnerable to breakage, loss by erosion, and reduced insert retention force due to less grip volume for resistance to rotation and dislodging forces. The required mud grooves defined in the cone created the need for additional erosion inserts to guard the roots of the cutting inserts, which in many cases were lost in any case due to root undercutting inherent in the mud flow along the grooves. When drilling, with a three cone rotary drill bit, the required weight on the drill string (as high as 75,000 pounds) is directly communicated to the drill bit cone shells and their cutting structure(s) as it rotates on the bottom of the hole being drilled. In traditional three cone rotary drill bits the larger diameter cones require radial clearance grooves to be defined in the cones surface in order to provide clearance for the cutting structure(s) of the adjacent cones. The required clearance grooves subsequently create small, and highly loaded, radial ribs, that serves as the load bearing surface area (riding on the hole bottom) which also serves as the insert retention area/cutting structure support area. By reducing the cone shell surface area in contact with the hole bottom to radial ribs (as a result of the required radial clearance grooves) the area in contact sees significantly higher unit loads which in turn causes accelerated wear. The required radial clearance grooves remove a substantial amount of material from the cones cross section further weakening the cone shell. The required radial clearance grooves also have another detrimental effect on the remaining radial ribs. As the cones rotate on the wellbore bottom (riding on the radial ribs), debris are entrapped in the clearance grooves and a portion of these debris are extruded out of the grooves and in between the inserts causing a powerful continuous erosive effect to the radial ribs/cutting structure support area/insert retention area additionally accelerating the rate of wear in this area. The resulting accelerated wear and wash-out of the remaining ribs undermines the insert retention area/cutting structure support area causing a loss of retention area, retention force, and ultimately loss of the cutting structure itself. With the reduction in support material the TCIs (tungsten carbide inserts) rotate, break, and dislodge causing the drill bit to fail prematurely. As an attempt to correct this condition, builders of conventional three cone rotary drill bits, add small “protection inserts” to the remaining radial ribs surrounding the cutting inserts with little or no positive results.
Radii of the leg-to-leg journal is limited in the conventional design thereby limiting journal strength and load capability.
Cutting inserts are press fitted into conventional cones, which limits the insert grip force and imposes damaging shear forces on the insert hole walls and exposes the unsupported portion of the cutting insert to high press forces during insert installation potentially causing micro fissures in the insert leading to early field failures.
The fabrication method of the leg/body segments which are three pieces welded together to form the bit body of conventional designs creates misalignments which causes the details of geometry of each bit to be individualized or untrue to varying degrees.
Conventional rotary cone bits include a short-travel rubber equalizer diaphragm in the grease loop that is directly exposed to the drilling environment which is easily subject to tampering. The conventional grease filling procedure entraps air in the bearing zones of the bit, the entrapped air compresses as the bit travels down hole due to increasing atmospheric pressure due to increasing mud weight thereby causing the equalizer to go the full length of its short travel or compensation prematurely, resulting in the failure of the equalizing lubrication system for the bearing.
The critical bearing and abrading surfaces of conventional three cone drill bits are typically uncoated and have only the friction resistance, hardness, and toughness, of the parent and/or wear pad material which may be heat treated and/or case hardened.